In recent years, increase of transmission capacity has been required due to sharp increase of communication traffic. An optical communication module is a key device of an optical network system. As a system becomes high-speed and large-capacity, it is required to make an optical communication module small-sized and high-speed.
A light-receiving module for optical communication generally has an optical fiber that transmits optical signals, a PD (photodiode) that converts the optical signals transmitted through a transmission path into electric current, and a TIA (transimpedance amplifier) that performs impedance conversion and amplifies the current signals and outputs as voltage signals, which are packaged in a ceramic package.
There are an increasing number of techniques for making an optical communication module high-speed by making the inside of the module multi-channel, for example, realizing 40 Gbps with 10 Gbps×4 or realizing 100 Gbps with 25 Gbps×4.
In order to make a light-receiving module high-speed, it is necessary to mount electric components on the same board and make GND common for decrease of loss of electric signals, and make the interconnection length between components as short as possible. Thus, as a multi-channel high-speed light-receiving module like a digital coherent receiver module, a structure that bends multi-channel interference signal light generated on a PLC and causes the light to enter into a PD mounted on the same carrier as a TIA and a ceramic circuit board is widely known.
Further, as a light-receiving module, there is a structure that is equipped with a monitoring PD and monitors the level of signals via a tap prism to check the intensity of transmitted signal light. In the case of inserting an optical component such as a prism in an optical path, it is general to use a collimation optical system, because component characteristics do not become stable when light is diffusing or converging.
Since a light-receiving module does not have an element serving as a light source, it is general to cause light to enter from the side of an optical fiber, and actively implement while checking a PD current value. Depending on the types and structures of the components mounted inside, it is possible to use, as a light source, feeble light of a PD that illuminates when PD forward current is flown, and thereby determine a position to mount a lens for collimating light.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 1988-139307
However, a light-receiving module structured to branch signal light into multiple channels has a problem that, from feeble light obtained by PD forward current, a sufficient amount of light to monitor cannot be obtained due to propagation loss up to the incident end. Therefore, it is considered to implement by causing a laser beam to enter in a direction opposite to an optical path and monitoring a large amount of dummy light. However, in a light-receiving module having a structure that an optical fiber is placed outside a package, there is no way to cause the dummy light to enter because the exterior wall of the package blocks the dummy light. That is to say, there is a problem that it is difficult to position a lens in a light-receiving module.
In Patent Document 1, a technique of regulating an optical axis by providing a dummy light source in a light-emitting module structured so that a light ray from a light source directly enters into a lens system is described as a related technique. However, since the exterior wall of a package becomes an obstacle in a light-receiving module as mentioned above, it is difficult to apply the technique described in Patent Document 1.